Method and apparatus for performing gamut mapping between heterogeneous devices

ABSTRACT

Provided is a method and apparatus to perform gamut mapping between heterogeneous devices, more particularly, a method and apparatus to map a color of a device to an identical or visually and sensually preferable color of another device having a different color gamut. The method includes adjusting lightness of a color of a source device by scaling to match a lightness range of the source device with a lightness range of a destination device; modifying the adjusted lightness of the color of the source device by adjusting a color gamut of the source device to a color gamut of the destination device; and to map the modified color of the source device to a color in the color gamut of the destination device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No.10-2006-0096300 filed on Sep. 29, 2006 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a method and apparatus to perform gamutmapping between heterogeneous devices, and more particularly, to amethod and apparatus to map a color of a device to an identical orvisually and sensually preferable color of another device having adifferent color gamut.

2. Description of the Related Art

Color input/output devices, which reproduce colors, such as monitors,cameras and printers, use different color spaces or models according tofields in which they are used. For example, in the case of a colorimage, printing devices use a cyan, magenta and yellow (CMY) colorspace, or a cyan, magenta, yellow and black (CMYK) color space whilecolor cathode ray tube (CRT) monitors or computer graphics devices use ared, green and blue (RGB) color space. In order to define adevice-independent color, which can be accurately reproduced anywhereregardless of devices, a CIE color space may be used. Major examples ofthe CIE color space include CIE-XYZ, CIE-Lab, CIE-Luv and CIECAMO2.

Apart from the color spaces, the color input/output devices may alsohave different ranges of reproducible colors, i.e., different colorgamuts. Due to such differences in color gamut, the same image may lookdifferent from one color input/output device to another. Therefore, if acolor signal is received from a source device having a different colorgamut from that of a destination device which will reproduce the inputcolor signal, it is required to appropriately convert the received colorsignal to match the color gamuts of the source and destination devices.This process is called “gamut mapping.”

For gamut mapping between, for example, a display and a color printer,the International Color Consortium (ICC), which is a color managementstandardization group, has standardized a technique of using a differentgamut mapping method according to a rendering intent. The ICC recommendsthat a hue preserved minimum Delta E (HPMINDE) gamut mapping methodshould be used for a relative colormetric intent and that a sigmodialgaussian lightness mapping, cusp & knee (SGCK) gamut mapping methodshould be used for a perceptual intent.

However, if a natural color image on a display is output by a printer,which is a destination device, using the HPMINDE gamut mapping method orthe SGCK gamut mapping method, the natural color image is distorted.

SUMMARY

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be apparentfrom the description, or may be learned by practice of the invention.

It is an aspect of the present invention to enable a destination deviceto reproduce the color of a source device as similar as possible to theoriginal color of the source device by adjusting the color gamut of thesource device to that of the destination device.

It is another aspect of the present invention to prevent the generationof contours and paling of a pure color.

It is another aspect of the present invention to enable a destinationdevice to output a visually and sensually preferable image by adjustingthe shape of the color gamut of a source device and the compression ratefor gamut mapping according to hue characteristics.

However, the aspects of the present invention are not restricted to theone set forth herein. The above and/or other aspects of the presentinvention will become more apparent to one of daily skill in the art towhich the present invention pertains by referencing a detaileddescription of the present invention given below.

According to an aspect of the present invention, there is provided amethod of performing gamut mapping between heterogeneous devices. Themethod includes scaling and thus adjusting lightness of a color of asource device in order to match a lightness range of the source devicewith a lightness range of a destination device; modifying the adjustedlightness of the color of the source device by adjusting a color gamutof the source device to a color gamut of the destination device; andmapping the modified color of the source device to a color in the colorgamut of the destination device.

According to another aspect of the present invention, there is providedan apparatus to perform gamut mapping between heterogeneous devices. Theapparatus includes a lightness adjustment module to adjust lightness ofa color of a source device by scaling to match a lightness range of thesource device with a lightness range of a destination device; a colorgamut modification module to modify the adjusted lightness of the colorof the source device by adjusting a color gamut of the source device toa color gamut of the destination device; and a gamut mapping module tomap the modified color of the source device to a color in the colorgamut of the destination device.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee. These and/or other aspects and advantages of theinvention will become apparent and more readily appreciated from thefollowing description of the embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 illustrates a conventional hue preserved minimum delta E(HPMINDE) gamut mapping method;

FIG. 2 illustrates a conventional sigmodial gaussian lightness mapping,cusp & knee (SGCK) gamut mapping method;

FIG. 3 is a flowchart illustrating a method of compensating a shadowregion according to an embodiment of the present invention;

FIGS. 4A and 4B illustrate a sigmoid function and the distribution ofweighting factors, which are used to scale lightness of a source device;

FIG. 5 illustrates a color gamut of the source device which was adjustedby scaling lightness of the source device;

FIG. 6 illustrates a color gamut of the source device which was modifiedby adjusting a cusp in the adjusted color gamut of the source device;

FIG. 7 illustrates a color gamut of the source device modified using anoffset;

FIG. 8 illustrates an example of applying a different offset accordingto a hue;

FIG. 9 illustrates a gamut mapping process according to an embodiment ofthe present invention;

FIG. 10 illustrates an example of applying a different knee lineaccording to a hue;

FIG. 11 illustrates color gamut boundaries obtained using differentgamut mapping methods;

FIGS. 12A-12D illustrate images output using different gamut mappingmethods; and

FIG. 13 is a block diagram of an apparatus to perform gamut mappingbetween heterogeneous devices according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art. Likereference numerals in the drawings denote like elements, and thus theirdescription will be omitted.

FIG. 1 illustrates a conventional hue preserved minimum delta E(HPMINDE) gamut mapping method.

Generally, a color gamut of a display is wider than that of a printer asillustrated in FIG. 1. Hence, not all colors from the display can beoutput by the printer. In the HPMINDE gamut mapping method, some ofcolors reproducible by the display, which fall outside the color gamutof the printer, are mapped to those having minimum color differencesamong colors reproducible by the printer. For example, referring to FIG.1, a color of the display is outside the color gamut of the printer. Ifthe color of the display is moved to a color having a minimum colordifference, it may be mapped to a boundary line of the color gamut ofthe printer. However, if the color of the display is within the colorgamut of the printer, it is not mapped and is maintained unchanged.

A drawback of the HPMINDE gamut mapping method is that colors in aregion of the display, such as a hatched region in FIG. 1, is mapped toone gamut mapped color. Therefore, different colors on the display areoutput as the same color by the printer, thereby causing contours in anoutput image.

FIG. 2 illustrates a conventional sigmodial gaussian lightness mapping,cusp & knee (SGCK) gamut mapping method.

The conventional SGCK gamut mapping method may include an operation ofscaling lightness of a color of a display and an operation of mappingthe color with the scaled lightness.

Generally, a printer and a display have different lightness ranges.Therefore, the lightness of a color of the display color is scaled andthus adjusted in order to match the lightness range of a color gamut ofthe display with that of a color gamut of the printer.

In the color mapping operation, a color of the display may be compressedbased on an anchor point 220. A knee line 210 is set at a positioncorresponding to 90% of a distance from the anchor point 220 to a colorgamut boundary of the printer. If a scaled display color is inside theknee line 210, it is maintained unchanged. However, if the scaleddisplay color is outside the knee line 210, it may be mapped using acompression technique. The knee line 210 denotes a line formed byconnecting points located at a position corresponding to a percent (%)version of the distance between the anchor point 220 and the color gamutboundary.

If a pure color with high chroma is reproduced using the SGCK gamutmapping method, it may be mapped to a color with low chroma. Inparticular, if pure colors around a cusp 200 in the color gamut of thedisplay are mapped using the SGCK gamut mapping method, the printer mayoutput colors with relatively significantly lower chroma. In otherwords, paled pure colors may be reproduced.

FIG. 3 is a flowchart illustrating a method of performing gamut mappingbetween heterogeneous devices according to an embodiment of the presentinvention.

Referring to FIG. 3, the method includes an operation of converting acolor of a source device into a color in a color space having lightness,chroma and hue data (operation S300), an operation of scaling thelightness of a modified color based on a lightness range of adestination device (operation S310), modifying the scaled lightness ofthe color of the source device using a color gamut of the destinationdevice (operation S320), an operation of mapping the color with themodified lightness to a color of the destination device (operationS330), and an operation of converting the color of the destinationdevice into a color in a color space which can be output by thedestination device (operation S340).

In the operation of converting the color of the source device into thecolor in the color space having lightness, chroma and hue components(operation S300), an input color signal of the source device isconverted from a device-dependent color space, such as a red, green andblue (RGB) color space or a cyan, magenta, yellow and black (CMYK) colorspace, to a device-independent color space, such as a CIE-Lab colorspace, and then to a lightness, chroma and hue (LCH) coordinate system.

Specifically, an input color signal may be converted from the RGB orCMYK color space to the CIE-Lab color space. For such conversion, astandard chart may be color-measured using a colorimeter. Then, amapping table between RGB or CMYK hues of the source device and CIE-Labhues, which were color-measured by the colorimeter, may be created. Eachof the CIE-Lab hues consists of luminosity, i.e., a lightness element L,and two tone elements a and b. Element a is positioned between green andred, and element b is positioned between blue and yellow.

After the RGB or CMYK color space is converted into the CIE-Lab colorspace, the CIE-lab color space may be converted into the LCH color spaceusing a Lab value of the CIE-Lab color space as defined by Equation (1).

C=√{square root over (a² +b ²)}

H=tan⁻¹(b/a)   (1)

where C indicates chroma, and H indicates hue. In the case of lightness,a value of the lightness element L, which represents lightness of Lab,may be used.

In a similar way, input RGB or CMYK data of the source device may beconverted into JCh data having lightness, chroma and hue components. Acolor space having the JCh data is called a CIECAMO2 color space. Inorder to convert the RGB or CMYK data into the JCh data, the RGB or CMYKdata is converted into XYZ data using the colorimeter, and then the XYZdata is converted again into the JCh data. For more detailed conversionprocess, “IEC TC-100, IEC 61966-2-1, Color Management Default RGB ColorSpace sRGB (1999)” may be referred to.

In the operation of scaling the lightness of the modified color based onthe lightness range of the destination device (operation S310), thelightness range of the source device is scaled to that of thedestination device, thereby adjusting the lightness of the color of thesource device.

If the lightness range of the source device is wider than that of thedestination device, the lightness of the color of the source device isscaled down to be within the lightness range of the destination device.Such scaling may be performed using Equation (2).

L _(sc)=(1−p _(c))L _(or) +p _(c) L _(s)   (2)

where L_(sc) indicates scaled and adjusted lightness, L_(or) indicateslightness of the color of the source device, and L_(s) indicateslightness into which the lightness of the color of the source device wasconverted using a sigmoid function. In addition, p_(c) indicates aweighting factor and may be given by Equation (3).

$\begin{matrix}{p_{c} = {1 - {\sqrt{\frac{C^{3}}{C^{3} + {500\text{,}000}}}.}}} & (3)\end{matrix}$

According to Equations (2) and (3), as chroma is closer to zero, L_(sc)becomes closer to L_(s), and as chroma increases, L_(sc) becomes moreaffected by L_(or). In other words, as chroma is closer to zero,lightness is enhanced, and as chroma increases, the original lightnessof an image is maintained. For example, if the lightness range of thesource device is scaled using the sigmoid function and the weightingfactor p_(c) illustrated in FIGS. 4A and 4B, the lightness range of thecolor gamut of the source device is adjusted to that of the color gamutof the destination device as illustrated in FIG. 5. Then, the lightnessof the color of the source device may be adjusted to be within thelightness range of the color gamut of the destination device usingEquation (2).

After the scaling operation (operation S310), a height (lightness) of acusp of the source device is matched with that of a cusp of thedestination device, thereby modifying the adjusted lightness of thecolor of the source device (operation S320). The cusp denotes an apexhaving highest chroma in a color gamut.

The color gamut of the source device may be modified by adjusting theposition of the cusp of the source device to the position (lightness) ofthe cusp of the destination device. Accordingly, the lightness of thescaled color of the source device is modified. Such modification isperformed to make a hue distribution of the source device similar tothat of the destination device so that both devices produce similarcolor senses. Referring to FIG. 6, as a cusp 200 of the source device isadjusted, the color gamut of the source device is also adjusted.Therefore, a resultant color gamut of the source device has a similarshape to that of the color gamut of the destination device.

In another method of modifying the color gamut of the source device(operation S320), which is similar to the above method, an offset isused as illustrated in FIG. 7. Referring to FIG. 7, the lightnessposition of the cusp of the source device may be raised by the offsetfrom the lightness position of the cusp of the destination device. Suchadjustment is designed to prevent a pure color from being darklyreproduced when the pure color is mapped to a color of the destinationdevice.

By adjusting the position of the cusp of the source device, thelightness of the color of the source device, which was adjusted in thescaling operation, can be modified. The adjusted lightness of the colorof the source device may be modified using Equation (4).

$\begin{matrix}{{L_{mod} = {L_{sc} \times {ratio}}}{{{ratio} = {1 - {\left( {1 - \frac{L_{pr\_ cusp} + {offset}}{L_{sc\_ cusp}}} \right) \times \left( \frac{C_{sc}}{C_{sc\_ cusp}} \right)}}},}} & (4)\end{matrix}$

where L_(mod) indicates modified lightness, ratio indicates amodification ratio, L_(sc) _(—) _(cusp) indicates lightness of theadjusted cusp of the source device, and L_(pr) _(—) _(cusp) indicateslightness of the cusp of the destination device. In addition, C_(sc)indicates chroma of the source device, C_(sc) _(—) _(cusp) indicateschroma of the cusp of the source device, and offset indicates thedifference between the modified lightness of the cusp of the sourcedevice and the lightness of the cusp of the destination device.

The result of adjusting the color gamut of the source device using theoffset is illustrated in FIG. 7. When the color gamut of the sourcedevice is adjusted using the offset, the color of the source device canbe modified to become relatively brighter than when the color gamut ofthe source device is adjusted without using the offset.

The size of the offset may vary according to a hue. For example, if purecolors reproduced by a printer, i.e., the destination device, are dark,such as green, cyan, blue and magenta, the offset may be set to apredetermined positive number (for example, ten as illustrated in FIG.8). If the pure colors of the printer are bright, such as red andyellow, the offset may be set low (for example, zero as illustrated inFIG. 8). In addition, characteristics of each pure color of thedestination device may be identified, and the offset may be set as afunction. Then, different offsets may be applied to the pure colors,respectively.

FIG. 9 illustrates a process of mapping a modified color of a sourcedevice to a color gamut of a destination device.

Referring to FIG. 9, a point having equal lightness to that of a cusp ona color gamut boundary of the destination device and having zero chromais designated as an anchor point. A knee line is set at a positioncorresponding to N % of a distance from the set anchor point to thecolor gamut boundary of the destination device according to a hue.

If the modified color of the source device is inside the knee line, itis maintained unchanged. If the modified color of the source device isoutside the knee line, it may be mapped using Equation (5) below.

$\begin{matrix}{{d_{pr\_ color} = {{{{N(\%)}/100} \times d_{pr\_ gb}} + {\left( {100 - N} \right){(\%)/100} \times \frac{\left( {d_{di\_ color} - {{{N(\%)}/100} \times d_{pr\_ gb}}} \right)}{\left( {d_{di\_ gb} - {{{N(\%)}/100} \times d_{pr\_ gb}}} \right)}}}},} & (5)\end{matrix}$

where d indicates the distance between a point on a γ line and an anchorpoint. In addition, d_(di) _(—) _(color) indicates the distance betweena modified color of the source device on the γ line and the anchorpoint, d_(di) _(—) _(gb) indicates the distance between a modified colorgamut boundary of the source device on the γ line and the anchor point,d_(pr) _(—) _(gb) indicates the distance between the color gamutboundary of the destination device on the γ line and the anchor point,and d_(pr) _(—) _(color) indicates the distance between the anchor pointand a mapped color point on the γ line. The modified color of the sourcedevice, which falls outside the knee line, may be mapped to a colorlocated between the color gamut boundary of the destination device andthe knee line using Equation (5) and the compression technique.

The setting of the knee line represents a compression range of the colorgamut of the source device. For example, if the knee line is set to aposition corresponding to 100% of the distance from the anchor point tothe color gamut boundary of the destination device, the set knee line isthe same as the color gamut boundary of the destination device. In thiscase, the color gamut of the source device is compressed, a clippingeffect can be obtained. Clipping denotes mapping a point outside thecolor gamut boundary of the destination device to a point at which avirtual line extending from the point outside the color gamut boundaryof the destination device to the anchor point intersects the color gamutboundary of the destination device. Therefore, if the knee line matchesthe color gamut boundary of the destination device and if colors outsidethe knee line are mapped to colors in the color gamut of the destinationdevice, the colors outside the knee line are all mapped to the colorgamut boundary of the destination device.

On the other hand, if the knee line is set to a position correspondingto 20% of the distance from the anchor point to the color gamut boundaryof the destination device, a region inside the knee line is formed byconnecting points located at the position corresponding to 80% of thedistance from the anchor point to the color gamut boundary of thedestination device. Therefore, the modified color of the source deviceoutside the knee line can be mapped to a color between the knee line andthe color gamut boundary of the destination device, that is, 20% of thedistance from the color gamut boundary of the destination device to theanchor point.

Therefore, as illustrated in FIG. 9, three points representing modifiedcolors of the source device may be mapped respectively to colors withinthe color gamut of the destination device. For mapping, the distanced_(pr) _(—) _(color) between the anchor point and a coordinate point ofa mapped color may be calculated using Equation (5). Then, if a color ison the modified color gamut boundary of the source device, it may bemapped to the color gamut boundary of the destination device. If themodified color of the source device is on the color gamut boundary ofthe destination device, it may be modified to a color slightly outsidethe knee line.

In gamut mapping, a rate of the knee line may be vary according to a huein order to consider hue characteristics of the destination device. Forexample, if the conventional HPMINDE or SGCK gamut mapping method isused, too reddish skin color may be output. In addition, a green grassregion output by the destination device may be too saturated. In orderto perform gamut mapping on yellow between red and green, apredetermined rate of the knee line may be set. For example, N may be inthe range of 20% through 50%.

A blue region shows the greatest difference in color gamut when outputby the display and when output by the printer. Therefore, if thecompression technique is applied, the blue region may be darklyreproduced. However, if N is set to 100%, bright blue may be reproduced.As for a color between blue and yellow, the knee line may be adjustedusing a linear function so that the color can be naturally mapped. Inthis way, the rate of the knee line may be set differently by reflectingthe characteristics of each hue as illustrated in FIG. 10.

FIG. 11 illustrates color gamut boundaries obtained using differentgamut mapping methods.

Referring to FIG. 11, a color gamut boundary 1100 obtained using theHPMINDE gamut mapping method corresponds to an original color gamutboundary of a source device. In the HPMINDE gamut mapping method,lightness is not scaled. Since a color of the source device is mapped toa color having a minimum color difference, points outside a color gamutof a destination device are all mapped to a color gamut boundary of thedestination device. Furthermore, points around a cusp of the sourcedevice are all mapped to one cusp of the destination device, therebycausing contours. In the HPMINDE gamut mapping method, since colorsoutside the color gamut of the destination device are mapped to thecolor gamut boundary of the destination device, the colors outside thecolor gamut of the destination device are all expressed as a color onthe color gamut boundary of the destination device, which has a minimumcolor difference. Consequently, different colors from the source devicemay frequently be reproduced as the same color by the destinationdevice.

A color gamut boundary 1120 is obtained using the SGCK gamut mappingmethod. Since the color gamut boundary 1120 is adjusted by scalinglightness, a lightness range of the source device matches that of thedestination device. In the SGCK gamut mapping method, gamut mapping isperformed using the compression technique in color gamuts having thesame lightness range. Referring to FIG. 11, if the cusp of the sourcedevice is gamut-mapped using the SGCK gamut mapping method, it is mappedto a point at which a virtual line extending from the cusp of the sourcedevice to an anchor point meets the color gamut boundary of thedestination device. If the cusp of the source device, which correspondsto a pure color, is mapped, a paled pure color with significantly lowerchroma may be reproduced.

A color gamut boundary 1140 obtained using the gamut mapping methodaccording to the present invention is shaped relatively similar to thatof the destination device since the lightness of the source device isscaled and then the lightness of the cusp of the source device is setequal to or higher, by an offset, than that of the destination device.After the modified color gamut boundary 1140 of the source device isobtained, gamut mapping is performed using the compression technique andbased on the anchor point. Then, the destination device can represent asimilar grayscale to that of the source device. In particular, the cuspof the source device can be mapped to a color slightly brighter andhaving lower chroma than the cusp of the destination device. Therefore,a pure color, which is not dark and has relatively high chroma, can bereproduced.

Referring to FIG. 3, a mapped color is converted into a color in a colorspace which can be output by the destination device (operation S340).The mapped color corresponds to a point in the LCH or CIECAMO2 colorspace having lightness, chroma and hue components. Therefore, the mappedcolor can be converted into a value of the RGB or CMYK color spaceoutput by the destination device. This operation is a reverse operationto the operation of converting a color of the source device into a colorin the LCH color space.

FIG. 12A-2D illustrate images output using different gamut mappingmethods. FIG. 12A illustrates a display image, and FIG. 12B illustratesan image output using the conventional HPMINDE gamut mapping method. Inaddition, FIG. 12C illustrates an image output using the SGCK gamutmapping method, and FIG. 12D illustrates an image output using a gamutmapping method according to an embodiment of the present invention.

Referring to FIG. 12B, a converted color is concentrated in a regionwhere red fruit is output, thereby causing contours in the image. In theimage of FIG. 12C, a bright pure color with low chroma was not properlyreproduced. In particular, pure colors such as red, green, yellow andyellowish green were not properly expressed. In addition, a skin colorof a person reproduced in the images of FIGS. 12B and 12C is tooreddish.

In the image of FIG. 12D, pure colors, such as red, green, yellow andyellowish green, were reproduced to have high lightness and chroma, andnatural gradation without contours was expressed.

FIG. 13 is a block diagram of an apparatus to perform gamut mappingbetween heterogeneous devices according to an embodiment of the presentinvention.

Referring to FIG. 13, the apparatus includes a color space conversionmodule 1300, a lightness adjustment module 1310, a color gamutmodification module 1320, a gamut mapping module 1330, and a reversecolor space conversion module 1340.

The color space conversion module 1300 converts color data of a sourcedevice from an RGB or CMYK color space to an LCH or CIECAMO2 colorspace. In order to convert the RGB or CMYK color space into the LCHcolor space, the color space conversion module 1300 converts the colordata from the RGB or CMYK color space into a CIE-Lab color space andthen into an LCH coordinate system. Specifically, a mapping tablebetween RGB or CMYK hues of the source device and CIE-Lab hues, whichwere color-measured by a calorimeter, may be created. Then, after theRGB or CMYK color space is converted into the CIE-Lab color space, theCIE-lab color space may be converted into the LCH color space usingEquation (1).

In order to convert the RGB or CMYK color space into the CIECAMO2 colorspace, the RGB or CMYK data is converted into XYZ data using a mappingtable, which was color-measured by the colorimeter, and then the XYZdata is converted into JCh data. Consequently, lightness, chroma and hueof a corresponding color can be obtained.

The lightness adjustment module 1310 scales the lightness of a convertedcolor based on a lightness range of the destination device. Thelightness adjustment module 1310 may scale a lightness range of thesource device to that of the destination device using Equations (2) and(3). Consequently, the lightness adjustment module 1310 matches thelightness range of the color gamut of the source device with that of thecolor gamut of the destination device as illustrated in FIG. 5.

The color gamut modification module 1320 modifies the scaled lightnessof the color of the source device by matching the height (lightness) ofa cusp in a color gamut of the source device with that of a cusp of thedestination device. In another modification method, the color gamutmodification module 1320 may use an offset in order to prevent a purecolor from being darkly mapped. Specifically, the color gamutmodification module 1320 may set the height of the cusp of the sourcedevice higher than that of the cusp of the destination device by theoffset. Then, points including the cusp of the source device may bemultiplied by a ratio, which is calculated using Equation (4). As aresult, the scaled lightness may be modified. In addition, the size ofthe offset may be adjusted according to hue characteristics, therebyadjusting the color gamut of the source device.

The gamut mapping module 1330 maps points in the color gamut of thesource device modified by the color gamut modification module 1320,which are outside a knee line, to the color gamut of the destinationdevice based on an anchor point. If the color of the source device,which was modified by the color gamut modification module 1320, isinside the knee line, it is maintained the same. If the modified colorof the source device is outside the knee line, the distance from theanchor point to the color of the source device is calculated usingEquation (5). As a result, a color to which the modified color of thesource device is mapped can be obtained. The rate of the knee line maybe adjusted so that the degree of compression applied to gamut mappingcan be controlled according to a hue. The rate of the knee line may varyaccording to the hue characteristics.

The reverse color space conversion module 1340 converts points in amapped color space into points in the RGB or CMYK color space which canbe output by the destination device. This operation is a reverseoperation to the operation of converting the color data in the RGB orCMYK color space into a point in the LCH or CIECAMO2 color space, whichis performed by the color space conversion module 1300.

The term ‘module’, as used herein, means, but is not limited to, asoftware or hardware component, such as a Field Programmable Gate Array(FPGA) or Application Specific Integrated Circuit (ASIC), which performscertain tasks. A module may advantageously be configured to reside onthe addressable storage medium and configured to execute on one or moreprocessors. Thus, a module may include, by way of example, components,such as software components, object-oriented software components, classcomponents and task components, processes, functions, attributes,procedures, subroutines, segments of program code, drivers, firmware,microcode, circuitry, data, databases, data structures, tables, arrays,and variables. The functionality provided for in the components andmodules may be combined into fewer components and modules or furtherseparated into additional components and modules. In addition, thecomponents and the modules may be implemented to execute one or morecentral processing units (CPUS) in a device.

As described above, an aspect of the present invention provides at leastone of the following advantages.

First, since a color gamut of a source device is scaled to becomesimilar to that of a destination device, the destination device canexpress natural and smooth gradation.

Second, the generation of contours, which is a problem of theconventional HPMINDE gamut mapping method, and the reproduction of apaled pure color, which is a problem of the conventional SGCK gamutmapping method, can be prevented.

Third, since the size of an offset and a rate of a knee line areadjusted according to characteristics of each hue, the destinationdevice can output a generally accurate and visually and sensuallypreferable image.

However, the effects of the present invention are not restricted to theone set forth herein. The above and other effects of the presentinvention will become more apparent to one of daily skill in the art towhich the present invention pertains by referencing the claims of thepresent invention given below.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims. Theexemplary embodiments should be considered in descriptive sense only andnot for purposes of limitation.

1. A method of performing gamut mapping between heterogeneous devices,the method comprising: adjusting lightness of a color of a source deviceby scaling to match a lightness range of the source device with alightness range of a destination device; modifying the adjustedlightness of the color of the source device by adjusting a color gamutof the source device to a color gamut of the destination device; andmapping the modified color of the source device to a color in the colorgamut of the destination device.
 2. The method of claim 1, wherein theadjusting lightness of the color comprises adjusting the lightness ofthe color of the source device by combining the lightness of the colorof the source device and lightness into which the lightness of the colorof the source device was converted using a sigmoid function.
 3. Themethod of claim 1, wherein the modifying adjusted lightness of the colorof the source device comprises modifying the lightness of the color ofthe source device, which was adjusted in the adjusting lightness of thecolor, by adjusting the color gamut of the source device such that acusp of the color gamut of the source device can match a cusp of thecolor gamut of the destination device.
 4. The method of claim 1, whereinthe modifying adjusted lightness of the color of the source devicecomprises modifying the lightness of the color of the source device,which was adjusted in the adjusting lightness of the color, by adjustingthe color gamut of the source device such that the cusp of the colorgamut of the source device can be moved to a position higher than thecusp of the color gamut of the destination device by a predeterminedoffset in a lightness direction.
 5. The method of claim 4, wherein avalue of the predetermined offset varies according to a hue.
 6. Themethod of claim 1, wherein mapping the modified color comprises mappingthe modified color to an identical color if the modified color is insidea knee line of the destination device and mapping the modified color bycompressing the modified color in proportion to a distance between themodified color and an anchor point if the modified color is outside theknee line of the destination device.
 7. The method of claim 6, wherein arate of the knee line varies according to a hue.
 8. The method of claim1, further comprising: converting an input red, green and blue (RGB)color space or a cyan, magenta, yellow and black (CMYK) color space ofthe source device into a Lab color space; and converting the Lab colorspace into a lightness, chroma and hue (LCH) color space.
 9. The methodof claim 8, further comprising: converting a color from the LCH colorspace, which was mapped to be reproducible by the destination device,into the Lab color space; and converting the Lab color space into theRGB or CMYK color space.
 10. The method of claim 1, further comprisingconverting the input RGB or CMYK color space of the source device into aCIECAMO2 color space having JCh data which is defined by lightness,chroma and hue.
 11. The method of claim 10, further comprisingconverting the JCh data in the CIECAMO2 color space, which was mapped tobe reproducible by the destination device, into data in the RGB or CMYKcolor space.
 12. An apparatus to perform gamut mapping betweenheterogeneous devices, the apparatus comprising: a lightness adjustmentmodule to adjust lightness of a color of a source device by scaling tomatch a lightness range of the source device with a lightness range of adestination device; a color gamut modification module to modify theadjusted lightness of the color of the source device by adjusting acolor gamut of the source device to a color gamut of the destinationdevice; and a gamut mapping module to map the modified color of thesource device to a color in the color gamut of the destination device.13. The apparatus of claim 12, wherein the lightness adjustment moduleadjusts the lightness of the color of the source device by combining thelightness of the color of the source device and lightness into which thelightness of the color of the source device was converted using asigmoid function.
 14. The apparatus of claim 12, wherein the color gamutmodification module modifies the lightness of the color of the sourcedevice, which was adjusted by the lightness adjustment module, byadjusting the color gamut of the source device such that a cusp of thecolor gamut of the source device can match a cusp of the color gamut ofthe destination device.
 15. The apparatus of claim 12, wherein the colorgamut modification unit modifies the lightness of the color of thesource device, which was adjusted by the lightness adjustment module, byadjusting the color gamut of the source device such that the cusp of thecolor gamut of the source device can be moved to a position higher thanthe cusp of the color gamut of the destination device by a predeterminedoffset in a lightness direction.
 16. The apparatus of claim 15, whereina value of the predetermined offset varies according to a hue.
 17. Theapparatus of claim 12, wherein the gamut mapping module maps themodified color to an identical color if the modified color is inside aknee line of the destination device and mapping the modified color bycompressing the modified color in proportion to a distance between themodified color and an anchor point if the modified color is outside theknee line of the destination device.
 18. The apparatus of claim 17,wherein a rate of the knee line varies according to a hue.
 19. Theapparatus of claim 12, further comprising a color space conversionmodule to convert an input RGB color space or a CMYK color space of thesource device into a Lab color space and converting the Lab color spaceinto an LCH color space.
 20. The apparatus of claim 19, furthercomprising a reverse color space conversion module to convert a colorfrom the LCH color space, which was mapped to be reproducible by thedestination device, into the Lab color space and to convert the Labcolor space into the RGB or CMYK color space.
 21. The apparatus of claim12, further comprising a color space conversion module to convert theinput RGB or CMYK color space of the source device into a CIECAMO2 colorspace having JCh data which is defined by lightness, chroma and hue. 22.The apparatus of claim 21, further comprising a reverse color spaceconversion module to convert the JCh data in the CIECAMO2 color space,which was mapped to be reproducible by the destination device, into datain the RGB or CMYK color space.